A Claus unit is the primary processing system for recovering sulfur in a form of elemental sulfur from an acid gas stream containing hydrogen sulfide. Hydrogen sulfide gas occurs naturally in natural gas or is formed as a by-product in some gas processing systems. Hydrogen sulfide is highly toxic and requires removal and treatment from the gas stream. The need to efficiently process hydrogen sulfide and other sulfur containing compounds is important in reducing emissions to meet increasingly stringent fuel regulations and growing environmental concerns. Elemental sulfur is the ultimate state of recovery from sulfur containing species.
In a Claus unit, an acid gas feed stream containing hydrogen sulfide and a source of oxygen, such as air, are fed to a furnace. Acid gas feed streams have a wide range of compositions. Many acid gas feed streams originate from solvent absorption processes, such as amine absorption.
The absorption processes extract hydrogen sulfide from the by-product gases of petroleum refining, natural gas processing, and other industrial processes. Alternately, the acid gas feed stream can come from a sour water stripper unit.
Once fed to the furnace, the hydrogen sulfide undergoes partial combustion to form sulfur, sulfur dioxide, and water. To ensure efficient performance of the Claus reaction, the furnace temperature profile is maintained in the range of 850-1,200° C. To ensure full combustion of the contaminants, the temperature needs to be above 950° C. The temperature reached depends upon the other components present in the feed such as carbon dioxide (CO2), water (H2O), hydrocarbons, and other sulfur containing compounds. Combustible gases tend to increase temperature by means of combustion and inert gases tend to decrease temperature by means of dilution. Temperature is an important process parameter, because the conversion of hydrogen sulfide to sulfur is a function of temperature, with pressure playing a smaller role. The residence time in the furnace is typically on the order of 0.5 second.
The ratio of combustion products depends on the amount of oxygen available in the furnace. Other products formed in the furnace can include hydrogen, carbonyl sulfide, and carbon disulfide. The furnace also breaks down contaminants present in the acid gas stream, such as mercaptans.
The reaction products are fed to a first condenser, in which elemental sulfur condenses, is separated, and is collected in a sulfur pit, and the gaseous products are reheated and fed to a catalytic converter. The catalytic converter is maintained at an average temperature of about 305° C. The temperature is limited by the need to limit the exit temperature in order to avoid catalytic bed damages. In the catalytic converter, hydrogen sulfide reacts with sulfur dioxide in the presence of a catalyst and forms elemental sulfur and water. The products from the catalytic converter are fed again to a condenser in which elemental sulfur condenses and is collected in the sulfur pit. At the end of the process, an incinerator is included.
The heating, catalytic, and condensing stages can be repeated. In a conventional Claus unit, the stages are repeated a maximum of three times, with conversion of 96-98% of the hydrogen sulfide gas, depending on the feed composition. In a modified Claus unit, a tail gas treatment unit is included at the end of the process, between the final catalytic converter and the incinerator. In configurations with a tail gas treatment unit, the heating, catalytic, and condensing stages are usually only repeated twice. Modified Claus units with a tail gas treatment unit can reach up to 99.9% conversion of hydrogen sulfide into elemental sulfur. The tail gas treatment unit minimizes sulfur dioxide emissions from the incinerator.
To increase overall energy efficiency, heat capture processes can be combined with the Claus unit, such as steam generation in a waste heat boiler and the condensers.
Maintaining the temperature in the furnace is important to ensure the reaction to elemental sulfur and the destruction of other contaminants. As noted above, the furnace temperature is affected by the composition of the feed stream and significantly by the concentration of hydrogen sulfide in the feed stream. When the concentration of hydrogen sulfide in the acid gas stream is below about 55% by volume, the temperature profile in the furnace is reduced. The concentration of the hydrogen sulfide is affected by the amount of contaminants present in the acid gas. The type and amount of contaminants are affected by the source that generated the acid gas, and by the processing steps that produced the acid gas for processing in the Claus unit.
Contaminants in the feed stream include hydrocarbons. Hydrocarbons pose several problems for a Claus unit. First, it is not clear that hydrocarbons will fully combust because the C—H bond is generally stronger than the S—H bond, thus some hydrocarbons get carried through the furnace to the catalytic units. Second, the hydrocarbons have the potential to contribute to competing side reactions to produce carbon monoxide (CO), carbon disulfide (CS2), carbonyl sulfide (COS), and elemental carbon. Third, for some hydrocarbon combustion processes, such as those involving benzene, toluene, ethyl benzene, or xylene (BTEX or BTX), the furnace must be maintained at a high temperature. If the furnace temperature is too low due to a low concentration of hydrogen sulfide, the temperature may not be sufficient for BTEX degradation reactions to occur. BTX removal is important because BTX has a plugging effect on the catalyst in the catalytic converters. Catalyst poisoning from carbon compounds in the catalytic beds results in a loss of activity and high pressure drop requiring frequent catalyst rejuvenation and replacement.
Currently, several processes exist for addressing these concerns, but they all suffer from drawbacks. Acid gas bypass or split flow is used when the concentration of hydrogen sulfide is low in the feed stream. In a split flow operation, part of the stream is sent to the furnace of the Claus unit while the remaining portion of the stream is sent directly to the catalytic converters. Split flow suffers two drawbacks. First, there is an upper limit of two-thirds of the feed gas for bypass, requiring the furnace to operate under reducing conditions. Second, the increased presence of contaminants decreases the efficiency of the catalytic converters and contributes to catalytic deactivation and plugging.
Activated carbon can be used to separate BTEX from a process stream, but requires regeneration of the activated carbon, resulting in changing feed conditions to the Claus unit during the regeneration process, or shut down of the feed all together. Amine or solvent enrichment processes use solvent absorption to enrich the hydrogen sulfide concentration. Amine enrichment suffers because it introduces liquids to the process, requires significant maintenance, and requires a significant amount of energy for the operation and for the solvent regeneration.
Oxygen or air enrichment may enhance the flame temperature in the Claus unit, but requires expenditures for additional oxygen and oxygen recovery units. Natural Gas injection to the furnace can also enhance the temperature, but may increase the amount of contaminants entering the feed and components for potential competing side reactions and can increase the size of Claus unit. Methods of pre-heating the feed gas to ensure the temperature in the furnace require additional energy consumption adding substantially to the cost.
Therefore, a process which enriches the concentration of hydrogen sulfide in the acid gas stream and removes contaminants without requiring excessive amounts of energy, equipment and materials, or process shutdown is desired. Preferably, such a process, would maintain the overall sulfur capacity of the Claus unit, while increasing the overall sulfur recovery efficiency due to eliminated or reduced contaminants.